A method of recycling a recording medium such as paper by erasing the color of a toner image formed on the recording medium such as paper is very effective from the viewpoint of environmental protection and economic efficiency due to reduction in the using amount of a recording medium such as paper.
As an erasable toner, for example, as proposed in Japanese Patent No. 4105718, a toner which contains a color developable compound and a color coupler and the color of which can be erased by heating is known. In this technique, the color developable compound and the color coupler are melt-kneaded together with a binder resin by a kneading pulverization method, and the resulting melt-kneaded material is incorporated in the inside of a toner. With the use of this toner, by heating a sheet of printed paper to 100 to 200° C. for about 1 to 3 hours, the color of the printed image can be erased, and further, the paper on which the color of the printed image was erased can be recycled. This technique is an excellent technique capable of contributing to a decrease in environmental load by reducing consumption of paper.
However, when a kneading pulverization method is used, kneading is performed at a high temperature between about 100 and 200° C. under a high shearing force. Therefore, a leuco dye (the color developable compound) and the color coupler are uniformly dispersed in the binder resin and a reaction between the leuco dye and the color coupler is inhibited to decrease the developed color density of the toner. Further, if a toner material such as a binder resin or a release agent has a color erasing action, the developed color density of the toner is decreased during kneading in the same manner. Therefore, it is necessary to select toner materials with a low color erasing action. In particular, as a binder resin, only a specific resin with no color erasing action such as a styrene-butadiene resin is allowed to be used, and it is very difficult to use a polyester resin or a styrene-acrylic resin which is excellent in fixability because such a resin is liable to exhibit a color erasing action.
Accordingly, a toner which can satisfy all of the properties: fixability, color developability, and erasability was not obtained yet.